The increase in internet traffic and other telecommunications over the past several years has caused researchers to explore new ways to increase fiber optic network capacity by carrying multiple data signals concurrently through telecommunications lines. To expand fiber network capacity, fairly complex optical components have already been developed for wavelength division multiplexing (WDM) and dense wavelength division multiplexing (DWDM).
Planar lightwave circuit (PLC) technology is one technology that may be used to implement optical wavelength routers. In a PLC, light is restricted to propagate in a region that is thin in two dimensions, referred to herein as the transverse dimension and the lateral dimension, and extended in the other dimension, hi a conventional PLC, a core layer typically lies between a top cladding layer and a bottom cladding layer and channel waveguides are often formed by at least partially removing (typically through an etching process) core material beyond the transverse limits of the channel waveguide and replacing it with at least one layer of side cladding material that has an index of refraction that is lower than that of the core material. The side cladding material is usually the same material as the top cladding material.
Each layer is typically doped in a manner such that the core layer has a higher index of refraction than either the top cladding or bottom cladding. When layers of silica glass are used for the optical layers, the layers are typically deposited on a silicon wafer. Deposition processes include chemical vapor deposition (CVD), low pressure chemical vapor deposition (LPCVD), and/or plasma-enhanced CVD (PECVD). Moreover, one or more of the optical layers of the slab waveguide and/or channel waveguide may comprise an optically transparent polymer. For example, spin coating is one known film deposition method. A doped-silica waveguide is usually preferred because it has a number of attractive properties including low cost, low loss, low birefringence, stability, and compatibility for coupling to fiber.
The arrayed-waveguide grating router (AWGR) is an example of an integrated optical router. An AWGR is a PLC having at least one input channel waveguide, an input planar waveguide, an arrayed-waveguide grating (AWG), an output planar waveguide, and at least one output channel waveguide. Alternatively, an AWGR may comprise a plurality of output waveguides and a plurality of input waveguides. AWGRs may be configured to perform a variety of functions, for instance, they may function as multiplexers, demultiplexers, or they may be configured to perform both functions.
One aspect of performance that is affected by the present invention is referred to as polarization dependent wavelength (PDW). This term, as well as a number of related terms, will now be defined. Spectral transmissivity (in units of dB) is defined as the optical power (in units of dBm) of substantially monochromatic light that emerges from the fiber that is coupled to the input port minus the optical power (in units of dBm) of the light that enters the optical fiber that is coupled to the output port of the optical router. Spectral transmissivity is a function of the selected input port, the selected output port, the optical wavelength, and the polarization state of the incident light. When the incident light is in a polarization state called a “principal state of polarization,” the light will be in the same polarization state when it emerges from the device. For purposes of illustration only, the principle states of polarization are assumed to be independent of wavelength, input port and output port. It is understood that the invention is not so limited by this assumption.
Again, for the purposes of illustration only, it will be assumed that the two principal states of polarization are the so-called transverse electric (TE) and transverse magnetic (TM) polarization states. The TE polarization state has an electric field that is predominantly aligned in the transverse direction and the TM polarization state has an electric field that is predominantly aligned in the lateral direction. Again, the invention is not so limited to devices having these principle states of polarization.
Typically, for values of spectral transmissivity that are larger than −10 dB, the TM spectral transmissivity is a replica of the TE spectral transmissivity that is shifted in wavelength by an amount that is referred to as the polarization dependent dispersion (PDD). PDD is positive if the TM spectral transmissivity has a maximum that has a longer wavelength than the maximum of the TE spectral transmissivity and is negative otherwise. PDW is defined as the absolute value of the PDD.
In many fiber optic communication systems, the polarization state of the light in the optical fiber may change in a manner that is uncontrolled and unpredictable. A change in the polarization state of the light in the fiber as it enters an AWG will cause a change in the optical power that emerges from the AWG that may be as large as the value of polarization dependent loss (PDL) for the AWG. There have been a number of techniques developed in an attempt to minimize PDW.
One approach to minimizing PDW involves selecting an optical layer design with minimum birefringence. In one example of this approach, U.S. Pat. No. 5,930,439 (Ojha et al.), which is incorporated herein by reference in its entirety, discloses a planar optical waveguide which reduces birefringence by doping the various optical layers so that the top cladding has a thermal coefficient of expansion that is close to the thermal expansion coefficient of the substrate.
Another approach is described in A. Kilian et al., Birefringence Free Planar Optical Waveguide Made by Flame Hydrolysis Deposition (FHD) Through Tailoring of the Overcladding, Journal of Lightwave Technology, v. 18, no. 2, p. 193 (2000), which is incorporated herein by reference in its entirety. Kilian discloses that because the thermal expansion of the top cladding largely determines the birefringence in the waveguide, a top cladding can be developed and made with a flame-hydrolysis-deposition (FHD) process to reduce the birefringence.
Many methods and apparatus attempt to formulate a top cladding material which reduces birefringence. However, this may require specific compositions for the top cladding. These techniques, as well as others, teach the application of a specific top cladding composition to reduce birefringence. However, there is a need for a reliable method to reduce the birefringence in a waveguide for varying top cladding compositions.